Started the app, registered an account via the app and tried to add the devices to the new account, but tapping on the "+" symbol on top menu-bar always ends in crashing the app.

No way to do otherwise...Started and restarted, Disinstalled and reinstalled: no way to have the app functional...

2) Powered a single R4 board and connected the mob to the AP 10.0.1.1 as described in the instructions: connected the board to my actual AP of the home network via the onboard feature exposed on the page by the board,.

The DHCP-assigned IP in the home range is correctly pingable, also after several switch-off.

Anyway there's no way to reset this type of configuration to AP mode, nor to configure the device network with a static IP, not in DHCP. There is no specific function in the software.